1. Field of the Invention
The present invention relates to a light emitting diode (hereinafter referred to as an "LED") having a double heterojunction structure. More particularly, the present invention relates to a technique for preventing a reduction in a light output of an LED for long-time operation.
2. Description of the Related Art
An LED having a so-called double heterostructure has a high level of light emission efficiency and a high light output and therefore is widely used for a display, a light source of optical communications, or the like.
FIG. 12 is a cross-sectional view illustrating a conventional LED 800 having a typical double heterostructure. The LED 800 is an InGaAlP based LED which includes layers having lattice match with a GaAs substrate and emits light ranging from red light to green light. In the LED 800,
a substrate 1: made of n-type GaAs; PA1 a first buffer layer 2: made of n-type GaAs; PA1 a light reflection (DBR:Distributed Bragg Reflector) layer 3: including n-type (Al.sub.0.4 Ga.sub.0.6).sub.0.5 In.sub.0.5 P layers and n-type Al.sub.0.5 In.sub.0.5 P layers deposited in an alternative fashion; PA1 a first cladding layer 4: made of n-type Al.sub.0.5 In.sub.0.5 P, doped with Si at an impurity concentration of 5.times.10.sup.17 cm.sup.-3, 1 .mu.m thick; PA1 a light emitting layer 6: made of p-type (Ga.sub.0.7 Al.sub.0.9).sub.0.5 In.sub.0.5 P, 0.5 .mu.m thick; PA1 a second cladding layer 7: made of p-type Al.sub.0.5 In.sub.0.5 P, doped with Zn at an impurity concentration of 5.times.10.sup.17 cm.sup.-3, 1 .mu.m thick; PA1 a first current diffusion layer 91: made of p-type Al.sub.0.7 Ga.sub.0.3 Al, doped with Zn at an impurity concentration of 1.times.10.sup.18 cm.sup.-3, 1 .mu.m thick; and PA1 a second current diffusion layer 92: made of p-type Al.sub.0.7 Ga.sub.0.3 As, doped with Zn at an impurity concentration of 3.times.10.sup.18 cm.sup.-3, 6 .mu.m thick; PA1 a substrate 101: made of n-type GaAs; PA1 a buffer layer 102: made of n-type GaAs; PA1 an n-type first cladding layer 103: made of n-type (Ga.sub.0.3 Al.sub.0.7).sub.0.5 In.sub.0.5 P, doped with Si at an impurity concentration of 1.times.10.sup.18 cm.sup.-3, 1 .mu.m thick; PA1 a light emitting layer 104: made of p-type (Ga.sub.0.7 Al.sub.0.3).sub.0.5 In.sub.0.5 P, 0.5 .mu.m thick; PA1 a p-type second cladding layer 105: made of p-type Al.sub.0.5 In.sub.0.5 P, doped with Zn at an impurity concentration of 5.times.10.sup.17 cm.sup.-3, 1 .mu.m thick; PA1 a first current diffusion layer 61: made of p-type Ga.sub.0.3 Al.sub.0.7 Al, doped with Zn at an impurity concentration of 1.times.10.sup.18 cm.sup.-3, 1 .mu.m thick; PA1 a second current diffusion layer 62: made of p-type Ga.sub.0.3 Al.sub.0.7 As, doped with Zn at an impurity concentration of 3.times.10.sup.18 cm.sup.-3, 6 .mu.m thick; and PA1 a contact layer 108: made of p-type GaAs.
are deposited in this order.
The first and second current diffusion layers 91 and 92 constitute a current diffusion layer 9.
A film of AuGe is provided as an n-side electrode 11 on a lower surface of the substrate 1 by a typical deposition method. A film of AuZn is provided on a upper surface of the p-type current diffusion layer 9 by the same deposition method. The AuZn film is subjected to photolithography patterning so as to remain a circular portion thereof as a p-side electrode 10 to which a metal wire is bonded for connecting the p-side electrode 10 to an external conductor. Light generated in the light emitting layer 6 is radiated from a portion of the upper surface of the p-type current diffusion layer 9 from which the AuZn film has been removed.
The first buffer layer 2 is used for preventing defects and contaminants of the substrate 1 from effecting the layers deposited the substrate 1. The first buffer layer 2 is not required when the substrate 1 has a satisfactorily treated upper surface. The DBR layer 3 reflects light generated in the light emitting layer 6 toward the substrate 1. This prevents light absorption by the substrate 1 and the reflected light goes in a direction away from the substrate 1, contributing to the brightness of the LED 800.
The current diffusion layer 9 has low resistivity so as to make an approximate ohmic contact with the p-side electrode 10 and also to diffuse a current injected from the p-side electrode 10 into the entire light emitting layer 6. This is why the current diffusion layer 9 requires a high level of impurity concentration, in this case, to prevent impurity Zn from diffusing into the light emitting layer 6, the first current diffusion layer 91 having a low impurity concentration is provided in the lower part of the current diffusion layer 9.
To obtain a high level of light emission efficiency, a conventional LED adopts a double heterostructure as shown in FIG. 15. FIG. 15 is a cross-sectional view illustrating an example of an AlGaInP based LED 900 which have lattice match with GaAs substrate 101. A structure of each layer in the LED 900 is as follows:
An n-side electrode 109 and a p-side electrode 107 are provided on the substrate 1 and the contact layer 108, respectively.
The AlGaInP based LED 800 in FIG. 12 generates light by injecting a current. In FIG. 13, a dashed line A indicates a relationship between an impurity concentration of the light emitting layer 6 and a light output in an initial period after starting light emission. The peak of the light output is at an impurity concentration of 1.times.10.sup.17 cm.sup.-3 in the initial period after starting light emission. However, the light output gradually decreases with time. For example, a current of 50 mA is supplied to the LED 800 for 1000 hours at room temperature. In FIG. 13, a dashed line B indicates a relationship between an impurity concentration of the light emitting layer 6 and a light output after the 1000-hour light emission. A light output after the 1000-hour light emission becomes lower at an impurity concentration of 1.times.10.sup.17 cm.sup.-3 while a light output becomes higher at an impurity concentration of 5.times.10.sup.17 cm.sup.-3 where the light output is maximum, which is different from in the initial period after starting light emission.
Our studies have found that such a change in a light output after long-time light emission is caused by: (1) a non-radiative recombination center generated at a pn junction interface between the n-type first cladding layer 4 and the p-type light emitting layer 6; and (2) an influence from diffused impurities in the light emitting layer 6.
FIGS. 14A and 14B illustrate states of energy bands of around the light emitting layer 6. FIG. 14A shows a state in the initial period after starting light emission, while FIG. 14B shows a state after the long-time light emission.
The pn junction interface 40 is a heterointerface where two layers having largely different energy gaps As shown in FIG. 14A make contact with each other. There is a large internal stress at the heterointerface 40. When a voltage is applied between the p-side electrode 10 and the n-side electrode 11 in order to generate light, a high electric field level is applied across the heterointerface 40.
The combination of the internal stress and the energy of light generated in the light emitting layer 6 causes a lattice defect at the heterointerface 40. This lattice defect grows along the direction of the electric field line into the light emitting layer 6 over the long-time light emission. The lattice defeat leads to formation of a deep energy level 20 in the vicinity of the heterointerface 40 as shown in FIG. 14B. The carriers, a hole and an electron, combine together at the deep energy level without emitting light. Such a deep energy level is called a non-radiative energy level. Since radiative recombination 30 of the LED 800 is a spontaneous emission process, the non-radiative recombination 31 at the non-radiative energy level 20 has a shorter lifetime than that of the radiative recombination 30. Therefore, when the number of carriers combining at the non-radiative energy level 20 is increased, the light emission efficiency of the LED 800 decreases.
Long-time light emission continues to cause the growth of the lattice defect which becomes widespread inside the light emitting layer 6. In other words, the light emitting layer 6 develops a lot of portions having the non-radiative energy level 20. Therefore, the light emission efficiency of the LED 800 is further decreased, i.e., the light output of the LED 800 is reduced compared with in the initial period of the light emission.
Japanese Laid-Open Publication No. 2-151085 discloses a semiconductor light emitting device (hereinafter referred to as a "LED of conventional Example 2") having structure similar to that shown in FIG. 12. The LED of conventional Example 2 includes an intermediate cladding layers interposed between the light emitting layer 6 and the first and second cladding layers 4 and 7. The intermediate cladding layers each have a thickness of more than about 10 .ANG. and less than about 200 .ANG., and an energy gap having a value between those of the light emitting layer 6 and the first and second cladding layers 4 and 7. In the LED of conventional Example 2, heterointerfaces are formed between the intermediate cladding layer and the first and second cladding layers 4 and 7, and between the intermediate cladding layer and the light emitting layer 6. Accordingly, the differences in an energy gap at the interfaces can be decreased, thereby reducing the internal stress. This, therefore, creates difficulty for a lattice defect to be generated and thus there are less non-radiative recombination centers in the light emitting layer 6.
In the LED of conventional Example 2, however, a pn junction is formed at the interface between the light emitting layer 6 and the intermediate layer. A lattice defect due to light emission is generated at the interface where a high electric field level exists. Although a decrease in a light output of the LED of conventional Example 2 is effectively delayed, long-time light emission allows a lattice defect generated at the interface to develop. The growth of the lattice defect decreases a light output of the light emitting layer 6.
As described in FIG. 13, after the long-time light emission, the light emitting layer having a higher concentration of impurity will have a higher light output. This phenomenon will now be described. When the light emitting layer 6 has a higher concentration of impurity than an optimal concentration, the resistivity of the light emitting layer 6 becomes low. Therefore, an electric field applied across the pn junction interface between the first cladding layer 4 and the light emitting layer 6 becomes small in extent, resulting in a low light output in an initial period after starting light emission. After long-time light emission, extra impurities are diffused in the light emitting layer 6 because of the electric field and heat generated in the vicinity of the light emitting layer 6. The diffusion of impurities increases the electric field and therefore the light output. In this case, a defect is also generated at the pn junction interface, and therefore, the light emission efficiency decreases after the long-time light emission.
In the LED 900 shown in FIG. 15, the buffer layer 102 is used to shield the influence of defects and contaminants of the substrate 101. The buffer layer 102 is not necessary when surface treatment of the substrate 101 is satisfactory. The contact layer 108 is made of GaAs, which does not contain Al, in order to facilitate ohmic contact with the p-side electrode 107. The contact layer 108 does not allow light generated by the light emitting layer 104 to pass therethrough. However, the contact layer 108 is provided directly under the electrode 107, adding no disadvantage to light radiation.
In the LED 900 shown in FIG. 15, energy gaps of the light emitting layer 104 and the first and second cladding layers 103 and 105 are set by a molar fraction of Al. The lattice constant of a III-V compound semiconductor is almost not variable when Al is replaced with Ga or vice versa. The greater the molar fraction of Al that is included, the greater the energy gap of the compound semiconductor. Hereinafter, the proportion of Al in the total amount of Al and Ga in a mixed crystal is regarded as a molar fraction of Al in the mixed crystal.
To obtain a high light output of the LED 900, it is required to satisfactorily confine carriers within the light emitting layer 104 by making differences between the energy gaps of the light emitting layer 104 and the first and second cladding layers 103 and 105 sufficiently great. The LED 900 has a double heterostructure in which the (Ga.sub.0.7 Al.sub.0.3).sub.0.5 In.sub.0.5 P light emitting layer 104 is interposed between the n-type (Ga.sub.0.3 Al.sub.0.7).sub.0.5 In.sub.0.5 P first cladding layer 103 and the p-type (Ga.sub.0.3 Al.sub.0.7).sub.0.5 In.sub.0.5 P second cladding layer 105 which have great energy gaps. A molar fraction of Al of the light emitting layer 104 is 0.3 while both of molar fractions of Al of the first and second cladding layers 103 and 105 are 0.7.
To obtain a high light output of the LED 900, diffusion of carriers injected from the electrode 107 into the entire light emitting layer 104 is required. To this end, a decrease in the resistivity of the current diffusion layer 106 by increasing an impurity concentration of the current diffusion layer 106 to a sufficient high level is required. The substrate 101 is typically made of an n-type semiconductor, so that a p-type semiconductor is used for the current diffusion layer 106. However, an impurity for a p-type semiconductor, such as Zn or Mg, is likely to diffuse. An interface between the layers having impurity concentrations largely different from each other has a high impurity concentration gradient. Therefore, in the interface, the impurity is likely to diffuse due to interaction of electrical energy with light energy generated by the light emitting layer 104.
For example, the current diffusion layer 106 and p-type second cladding layer 105, as well as the p-type second cladding layer 105 and light emitting layer 104, have the above-described relationship therebetween. Therefore, impurity diffusion is likely to take place between the current diffusion layer 106 and p-type second cladding layer 105 and between the p-type second cladding layer 105 and light emitting layer 104.
Even when the light emitting layer 104 initially has an optimal concentration of a p-type impurity, the concentration changes due to diffusion of the impurity and therefore the light emission efficiency of the light emitting layer 104 is decreased. Further, the p-type impurity entering the light emitting layer 104 by diffusion is unlikely to settle into a normal position of lattice, becoming a non-radiative recombination center which has a deep energy level.
In the conventional LED 900 shown in FIG. 15, the current diffusion layer 106 includes two layers. The lower layer is a first current diffusion layer 61 having a low impurity concentration. Therefore, the impurity concentration gradient between the light emitting layer 104 and the first current diffusion layer 61 becomes a low value, whereby diffusion of Zn is prevented. The first current diffusion layer 61 and the second current diffusion layer 62 has the same molar fraction of Al.
Conventionally, molar fractions of Al of the first and second cladding layers 103 and 105 are about 0.7. The inventors have found that the above-described conventional technique is insufficient to prevent impurity diffusion when the molar fractions of Al of the first and second cladding layers 103 and 105 are increased up to about 1.0 in order to enhance carrier confinement and obtain a higher light output of the LED 900. In other words, the above-described p-type impurity diffusion is significant when a molar fraction of Al is great.